virsh: migrate --timeout-postcopy requires --postcopy

Requesting an automated switch to a post-copy migration (using
--timeout-postcopy) without actually enabling post-copy migration (using
--postcopy) doesn't really do anything. Let's make this dependency
explicit to avoid unexpected behavior.

https://bugzilla.redhat.com/show_bug.cgi?id=1455023Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

tools/virsh-domain.c

@@ -10768,6 +10768,7 @@ cmdMigrate(vshControl *ctl, const vshCmd *cmd)
     VSH_EXCLUSIVE_OPTIONS("live", "offline");
     VSH_EXCLUSIVE_OPTIONS("timeout-suspend", "timeout-postcopy");
     VSH_REQUIRE_OPTION("postcopy-after-precopy", "postcopy");
+    VSH_REQUIRE_OPTION("timeout-postcopy", "postcopy");
     VSH_REQUIRE_OPTION("persistent-xml", "persistent");
 
     if (!(dom = virshCommandOptDomain(ctl, cmd, NULL)))